Known optical security features include semi-covert images or watermark images applied to high-security documents such as banknotes. Watermark images have become well-known features for authenticating documents due in part to public familiarity and ease of use. Non-printed optical watermark images are introduced into selected areas of a document or substrate by varying a thickness of the substrate during a substrate manufacturing process. For example, a press having a desired texture pattern is rolled over the substrate to vary the thickness of the substrate according to the desired texture pattern. Variations in the substrate thickness modify translucent characteristics of the substrate, which may be viewed by holding the substrate against a light source or backlight to verify the presence of an anticipated watermark image.
One limitation of known non-printed optical watermarks is a lack of design complexity. Early non-printed optical watermarks were generally produced using a “dandy roll” manufacturing process that impresses a wet rolling stamp onto the paper prior to drying. These non-printed optical watermarks are prevalent in banknotes and are generally simple in design. The watermark designs often depict a portrait with minimal tonal or density variations. Another limitation of known non-printed optical watermarks is an inability to incorporate color into the watermark effect. Other non-printed optical watermarks utilized a “cylinder mould” manufacturing process that simulated true grayscale images using significant tonal and density variations. Both of these manufacturing processes are limited to producing grayscale images. In other words, these manufacturing processes are not capable of incorporating color into the watermark effect.
Additionally, any design that is printed on the front or back of the substrate should include a simple design to allow for optimal readability of the non-printed optical watermarks since it is difficult to align the printed image relative to the known non-printed optical watermarks. This is due to the watermark being created as a separate component from the printed image. In other words, the document printing does not occur “in-line” with generation of the non-printed optical watermarks. Therefore, additional measures are needed to ensure proper registration between the printed image and known non-printed optical watermarks.
Another limitation of non-printed optical watermarks is a lack of design integration flexibility. Since the non-printed optical watermarks are produced during the substrate manufacturing process, there is a limited opportunity to integrate a desired design into the substrate.
Yet another limitation of technology associated with known non-printed optical watermarks is general application to milled paper substrates. In other words, this technology has limited extension to paperless applications, such as polymer substrates and materials.
Additional limitations of known non-printed optical watermarks applied to banknote features is a low security level attributable to risk of simulation, process availability, and material chain of custody. For example, a substrate may include pre-printed images that simulate non-printed optical watermarks.
With respect to simulation of a non-printed optical watermark, the variable transparency introduced into known non-printed optical watermarks can be mimicked using a pre-printed document base substrate having an opaque ink that is printed prior to printing the visible document design. If a shade of ink is applied that is similar to a shade of the base substrate, the introduction of the pre-printed image may be difficult to detect; particularly after the pre-printed image is overprinted with a complex color design. The pre-printed image can include a negative image that, when backlit, reduces the transparency of the areas containing the opaque ink compared to the areas without the opaque ink. This effect can be controlled to appear in the form of a grayscale image that would be seen in non-printed optical watermarks. This method achieves a same level of covertness as non-printed optical watermarks. In fact, some polymer banknotes implement pre-printed image features intentionally, for use with transparent windows.
With respect to availability, manufacturing processes for producing known non-printed optical watermarks are becoming commonly available. There is no apparent regulation preventing entities other than accredited high-security printers with access to the manufacturing equipment. Therefore, known non-printed optical watermarks manufacturing processes are commercially available to standard non-security paper manufacturers.
With respect to material chain of custody, since known non-printed optical watermarks are contained within base substrates, extra measures are needed to ensure that all pre-manufactured material is accounted for and that any unused material is destroyed. Otherwise, counterfeiters having access to substrates with non-printed optical watermarks could pass off the substrate as an authentic substrate.